The present invention relates to the technical field of treatment of offgases, in particular the treatment of offgases containing nitrogen oxides.
The present invention relates in particular to a process for treating nitrogen oxide-containing offgases from industrial processes, e.g. flue gases, for the purposes of removing or separating out the nitrogen oxides and/or for the purposes of reducing the nitrogen oxide content by means of chemical reduction of the nitrogen oxides. In particular, the present invention relates to a process for removing nitrogen oxides from offgases from industrial plants, for example power stations, in particular combined heat-power stations, or waste incineration plants.
Furthermore, the present invention relates to an apparatus for treating nitrogen oxide-containing offgases from industrial processes, e.g. flue gases, for the purposes of removing or separating out the nitrogen oxides or else for the purposes of reducing the nitrogen oxide content by means of chemical reduction of the nitrogen oxides.
Combustion reactions in the presence of air form metastable, generally toxic and reactive oxides of nitrogen, known as nitrogen oxides. The formation of nitrogen oxides occurs to an increased extent as a result of the combustion or thermolysis and pyrolysis of organic and inorganic nitrogen-containing compounds, which occurs in large-scale firing plants such as combined heat-power stations or waste incineration plants.
Nitrogen oxides, in particular the compounds nitrogen monoxide and nitrogen dioxide known under the term nitrous gases, which are also denoted by the abbreviated formula NOx, are not only toxic and lead to irritation and damage to the pulmonary system but also increase the formation of acid rain since they react with moisture to form acids.
However, the liberation of nitrogen oxides is also problematical for further reasons of environmental protection since firstly they promote the formation of smog and of harmful ozone near the ground and secondly act as greenhouse gases and increase global warming.
Owing to the negative effects of nitrogen oxides on health and the environment and not least due to the economic damage associated therewith, attempts have been made for a long time to minimize or prevent the liberation of nitrogen oxides in combustion processes. In the case of passenger cars, this is achieved, for example, by the use of catalysts which allow virtually complete removal of the nitrogen oxides from the exhaust gases.
To reduce the emission of nitrogen oxides from industrial plants, in particular large industrial firing plants, various processes for nitrogen oxide removal or denitrification (deNOx) which alone or in combination are supposed to bring about an effective reduction or avoidance of nitrogen oxides in offgases, in particular flue gases, have been developed in view of the prevailing legal position and also economic considerations.
The processes and measures for reducing the nitrogen oxide content of offgases, in particular flue gases, can be divided into primary measures and secondary measures.
In the case of the primary measures, the combustion process is controlled in such a way that the nitrogen oxide content of the resulting offgases is as low as possible; the nitrogen oxides should, so to say, not be formed at all. Primary measures include, for example, flue gas recirculation, in which the flue gas is recirculated to the combustion zone, and also air and fuel stages in which the combustion is controlled in such a way that various combustion zones having different oxygen concentrations are obtained. In addition, the formation of nitrogen oxides in flue gases can also be reduced by addition of additives or by quenching, i.e. by spraying in water to reduce the temperature during the combustion process.
In contrast to primary measures, which are intended to reduce the formation of nitrogen oxides, the use of secondary measures is intended to reduce the concentration of the nitrogen oxides in the offgases, in particular flue gases. Secondary measures include, for example, separation processes in which the nitrogen oxides are chemically bound or scrubbed out of the flue gas stream. However, a disadvantage of the separation processes is that large amounts of waste products, for example process water, which are often contaminated with further constituents of the flue gas are obtained and have to be disposed of, which costs money.
For this reason, secondary measures employed in modern industrial plants are usually processes which are based on reduction of the nitrogen oxides to elemental nitrogen and lead to only small amounts of waste products, with a distinction generally being made between catalytic processes and noncatalytic processes.
Selective catalytic reduction (SCR) of the nitrogen oxides encompasses catalytic processes in which the nitrogen oxides are converted into elemental nitrogen with the aid of metal catalysts. In general, the best denitrification values are obtained by means of SCR processes, but the use of the catalyst makes the process significantly more expensive and less economically viable. In addition, the plants for carrying out the SCR process are extremely expensive not only to acquire but also to maintain since the sensitive catalysts have to be treated or replaced at short time intervals. Particularly in large firing plants whose fuel composition can often be determined only unsatisfactorily, for example waste incineration plants, there is always a risk of poisoning of the catalysts by impurities in the flue gas. This risk can be reduced only by means of additional costly measures.
Selective noncatalytic reduction (SNCR), on the other hand, is based on the thermolysis of nitrogen compounds, in particular ammonia or urea, which then react in a comproportionation reaction with the nitrogen oxides to form elemental nitrogen.
Compared to selective catalytic reduction, selective noncatalytic reduction is significantly cheaper to carry out: thus, the costs of acquiring and maintaining SNCR plants are just from 10 to 20% of the costs of corresponding SCR plants.
However, a problem with SNCR processes is that their effectiveness is less than the effectiveness of catalytic processes, so that, for example, in the event of a further reduction in the legally permitted limit values for nitrogen oxides in offgases, in particular flue gases, most SNCR plants will no longer be able to operate.
A further disadvantage of processes based on the selective noncatalytic reduction of nitrogen oxides is that the reducing agent has to be used in excess and does not react completely, so that the offgas contains an ammonia loading which is sometimes not insignificant. Excess ammonia in the offgas either has to be separated off or its content has to be reduced by means of process engineering measures to such an extent that release of the offgas stream into the environment is possible.
In addition, there are also processes which are based both on a catalytic mode of action and also on the use of reducing agents, but these processes, too, cannot overcome the in-principle disadvantages of the respective processes (high costs for the use of catalytic processes and low effectiveness for the use of reducing agents).